The many seed protein categories and the distribution of the F2 seeds into these categories can be better explained if we incorporate two ideas into our hypothesis.
1) Many pairs of genes control seed protein levels in soybean and the high protein and low protein parents have different alleles at some of these gene pairs. The number and frequencies of the possible F2 genotypes results in many protein phenotypes that appear in the observed frequencies.
2) Environment influences seed protein. Seeds with the same genetic makeup will vary slightly in their seed protein content as a result of where on the plant they develop and the plant’s growing conditions. This creates more seed protein categories.
The two ideas above explain the control of quantitative traits. Quantitative traits are usually traits that are measured, show continuous variation and are controlled by many genes plus environment. Therefore we could pose this hypothetical genetic explanation for seed protein variation in soybean.
The Aa, Bb, Dd, Ee, Ff, Gg, and H,h gene pairs work together to control seed protein levels.
The upper case alleles add to the seed protein level. The alleles have an additive gene action so that the upper case alleles are called plus or additive alleles and add the same average amount of protein. Drs. George Graef and Jim Specht are currently working on studies that may help them determine how many genes control seed protein and how much a single gene contributes to seed protein. In our hypothetical example, each upper case allele will add 2% more protein to the seed and the minimum amount of protein to make a soybean seed is 26%. Lower case alleles are neutral alleles and do not add protein. So at the A,a gene pair, AA is higher than Aa which is higher than aa.
The genotype that would give the highest possible protein is AABBDDEEFFGGHH (54%) and the aabbddeeffgghh genotype has the lowest possible seed protein (26%). A soybean with the genotype AABBDDeeffgghh would have the same seed protein (38%) as an aabbddEEFFGGhh plant, assuming they are grown in the same environment. The accounting of plus alleles and seed protein in these genotypes is given below: